the United States (Fig. 1). Nigeria is the world's largest cowpea producer, with production levels ranging between 40% (Rachie, 1985) and

75%(Cobley and Steele, 1977) of the world's output.

The cowpea plant is a herbaceous annual with habits ranging from

determinate to completely indeterminate types. The stem is nonpubescent

and may be erect, semierect, prostrate, or twining. The inflorescence is a

simple raceme borne on an elongated axillary peduncle on which the

flower opening is acropetal. The pod, a legume, contains seeds that are

arranged in a single row within the pod wall. The taproot system develops

both secondary and tertiary roots along its length, all of which are capable

of nodulation, hence symbiotic nitrogen fixation. The mutualistic interaction between the cowpea root and soil bacteria of the genus Rhizobium

results annually in an estimated production level of 198 kg/ha of nitrogen,

a level higher than that of Arachis hypogaea (124 kg/ha) or Glycine max

+ dispersal rouln

FIG. 1. Major cowpea-growing areas of the world, with West Africa as the center of

origin.

GENETIC MANIPULATION OF THE COWPEA

135

(103 kg/ha) (Nutman, 1975). The cowpea plant by this feat not only provides for its own nitrogen requirements but also contributes to the replenishment of nitrogen in tropical soils, and assists in cutting the everincreasing capital outlay on nitrate fertilization when intercropped with

tropical cereals. The cowpea plant is drought tolerant.

Vigna unguiculata is diploid with a chromosome number of 2n = 22. It

belongs to the tribe Phaseoleae of the subfamily Papilionoideae. It is

classified further into the subtribe Phaseolinae and group Phaseolastrae.

The genus Vigna is pantropic, comprising 7 subgenera and about 170

species (Cobley and Steele, 1977). The taxonomy of the cowpea at levels of

taxon below the generic appears complex.

Marechal’s approach to the classification (Marechal e t al., 1978;

Marechal, 1982) is given in Table I . Sexual incompatibility barriers exist in

the Phaseolinae and so far interspecific crosses are yet to be reported with